Catalytic activity of platinum supported on SnO2 in the reaction of CO oxidation by NO was examined. Catalysts were tested in gas mixture containing 1 vol.% of NO and 1 vol.% of CO in helium; activation procedure consisted of treatment of the catalysts with CO at 500°C. Improvement of catalytic activity after activation process was observed. The mechanisms of the oxidation and activation process with application of semiconductor resistive sensors made of Pt/SnO2 were investigated.